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It is a red solid. It is very unstable and a powerful reducing agent. It reacts violently with water. The reaction with water makes phosphorous acid and hydrogen iodide. It is used in making chemicals. Some think that it is too unstable to be stored, but it can be stored and can be bought.

Phosphorus(III) iodide is made by reacting iodine with white phosphorus dissolved in carbon disulfide. It is dissolved in carbon disulfide because it does not dissolve in water. It can also be made by reacting phosphorus(III) chloride with hydrogen iodide or some other iodide.